How to Restrict Numbers And Special Characters In Input Field Visualforce Page?

​Sample Code :

<apex:page standardController="Lead" docType="html-5.0">

    <apex:form>

        <apex:pageBlock>

            <apex:pageBlockSection>

                <apex:inputField id="firstname" value="{!Lead.FirstName}" onkeypress="return (event.charCode > 64 && event.charCode < 91) || (event.charCode > 96 && event.charCode < 123)" />

                <apex:inputField id="lastname" value="{!Lead.LastName}" onkeypress="return (event.charCode > 64 && event.charCode < 91) || (event.charCode > 96 && event.charCode < 123)" />

            </apex:pageBlockSection>

        </apex:pageBlock>

    </apex:form>

</apex:page>